how to install a testing package?

[hendrik@pooq.com]

I shouldn't have to ask this question, but I do.  It's probably all in
the documentation somewhere, but I'm a Debian newbie and I still can't
find my way around properly.

I have just installed the stable debian release woody from CD-ROMs, my
first experience with debian, and I have found it a success.  I 
especially liked app-get and its friends.

Now there's a package (twiki) in the testing distribution I'd like to
install.  It doesn't appear on the CDROMs, presumably because they are
just the stable release.

How do I go about it, so that it will be properly downloaded to the
proper place, installed properly, and I retain a copy of the package in
case I need to do it again?  Do I need to set up my own patial mirror on
my hard disk somewhere?

-- hendrik